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Unformatted text preview: ystem 15.8.1 Stability It is not feasible to calculate the spill current that may
occur, but, fortunately, this is not necessary; an alternative
approach provides both the necessary information and the
technique required to obtain a high performance. Network Protection & Automation Guide The current transformers are replaced in the diagram by
ideal current transformers feeding an equivalent circuit
that represents the magnetising losses and secondary
winding resistance, and also the resistance of
the connecting leads. These circuits can then be
interconnected as shown, with a relay connected to the
junction points to form the complete equivalent circuit.
Saturation has the effect of lowering the exciting
impedance, and is assumed to take place severely in
current transformer H until, at the limit, the shunt
impedance becomes zero and the CT can produce no
output. This condition is represented by a short circuit,
shown in broken line, across the exciting impedance. It
should be noted that this is not the equivalent of a
physical short circuit, since it is behind the winding
resistance .
Applying the Thévenin method of solution, the voltage
developed across the relay will be given by:
IR= Vf
R R + R LH + R CTH ...Equation 15.1 The current through the relay is given by:
= I f ( R LH + R CTH ) R R + R LH + R CTH ...Equation 15.2 If RR is small, IR will approximate to IF, which is
unacceptable. On the other hand, if RR is large IR is
reduced. Equation 15.2 can be written, with little error,
as follows: • 241 • Busbar P rotection The incidence of fault current with an initial unilateral
transient component causes an abnormal buildup of
flux in a current transformer, as described in Section
6.4.10. When throughfault current traverses a zone
protected by a differential system, the transient flux
produced in the current transformers is not detrimental
as long as it remains within the substantially linear range
of the magnetising characteristic. With fault current of
appreciable magnitude and long transient time constant,
the flux density will pass into the saturated region of the
characteristic; this will not in itself produce a spill
output from a pair of balancing current transformers
provided that these are identical and equally burdened.
A group of current transformers, though they may be of
the same design, will not be completely identical, but a
more important factor is inequality of burden. In the
case of a differential system for a busbar, an external
fault may be fed through a single circuit, the current
being supplied to the busbar through all other circuits.
The faulted circuit is many times more heavily loaded
than the others and the corresponding current
transformers are likely to be heavily saturated, while
those of the other circuits are not. Severe unbalance is
therefore probable, which, with a relay of normal burden,
could exceed any acceptable current setting. For this
reason such systems were at one time always provided
with a time delay. This practice is, however, no longer
acceptable. • 15 • IR I
Vf
=
=
RR f (R LH RL + RCT = lead + CT winding resistance + R CTH ) K RR … Equation 15.3 or alternatively:
I RRR = V f = I f ( R LH + R CTH ) It remains to be shown that the setting chosen is
suitable.
…Equation 15.4 It is clear that, by increasing RR, the spill current IR can be
reduced below any specified relay setting. RR is frequently
increased by the addition of a seriesconnected resistor
which is known as the stabilising resistor. Busbar P rotection It can also be seen from Equation 15.4 that it is only the
voltage drop in the relay circuit at setting current that is
important. The relay can be designed as a voltage
measuring device consuming negligible current; and
provided its setting voltage exceeds the value Vf of
Equation 15.4, the system will be stable. In fact, the
setting voltage need not exceed Vf, since the derivation
of Equation 15.4 involves an extreme condition of
unbalance between the G and H current transformers
that is not completely realised. So a safety margin is
builtin if the voltage setting is made equal to Vf. • 15 • = factor depending on relay design
(range 0.7  2.0) The current transformers will have an excitation curve
which has not so far been related to the relay setting
voltage, the latter being equal to the maximum nominal
voltage drop across the lead loop and the CT secondary
winding resistance, with the maximum secondary fault
current flowing through them. Under inzone fault
conditions it is necessary for the current transformers to
produce sufficient output to operate the relay. This will
be achieved provided the CT kneepoint voltage exceeds
the relay setting. In order to cater for errors, it is usual
to specify that the current transformers should have a
kneepoint e.m.f. of at least twice the necessary setting
voltage; a higher multiple is of advantage in ensuring a
high speed of operation. It is necessary to realise that the value of If to be inserted
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 Spring '13
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